1. Field of the Invention
The present invention relates to methods for controlling a switching power supply including a plurality of switching elements and, more particularly, to control methods that do not require an oscillation circuit.
2. Description of the Related Art
Control procedures called PWM (Pulse Wide Modulation) systems and PFM (Pulse Frequency Modulation) systems are generally available as methods for controlling switching elements in switching power supplies (See Non-Patent Document 1).
PWM systems are systems for controlling the ratio of an ON-period of a switching element to a switching period, and generally, the switching period is constant. In a case where a plurality of switching elements is provided, the ON-period ratios of the switching elements are equal to each other or inverted with respect to each other.
PFM systems are systems for controlling a switching frequency, and generally, an ON-period ratio of a switching element is constant. In a case where a plurality of switching elements is provided, the relationships between the ON-period ratios and the switching frequencies of the switching elements are equal to each other. Non-Patent Document 1: Electrical Engineering Handbook (Sixth Edition) by The Institute of Electrical Engineers of Japan, Feb. 20, 2001, Vol. 20, Chapter 9, Section 2 Switching Regulator, pp 851-852.
In known technologies, when a plurality of switching elements is provided, an oscillation circuit is also provided. A plurality of driving signals is produced based on an oscillation signal of the oscillation circuit, and the driving signals are transmitted to control terminals of the switching elements. Thus, if a delay time or an advanced time is generated in a path for transmitting the driving signals or a driving circuit, a phenomenon in which the plurality of switching elements is in an ON-state at the same time occurs even though the plurality of switching elements connected in series with each other must be driven in order. This phenomenon may not only prevent normal operation but may also destroy a power-supply unit due to an overcurrent or the like, thus significantly reducing reliability.
Thus, in order to avoid the phenomenon of the plurality of switching elements being in the ON-state at the same time, a dead time in which the plurality of switching elements is in an OFF-state at the same time is provided. However, since the dead time does not contribute to voltage conversion, providing a dead time having an unnecessary long duration is one factor that reduces the power conversion efficiency. In addition, since the ON-period ratio and the switching frequency change in PWM systems and PFM systems, respectively, it is very difficult to set the dead time properly, and a complicated configuration is required.
In addition, in known technologies, obviously, an oscillation circuit functioning as a standard is required.
Furthermore, in known technologies, a control process to stabilize an output voltage by changing an ON-period of a switching element functioning as a standard is performed. For example, keeping an output voltage constant is only one condition for the control.